In NPLs 1 and 2, a technique referred to as OpenFlow has been proposed. OpenFlow recognizes communications as end-to-end flows and performs path control, failure recovery, load balancing, and optimization on a per-flow basis. Each OpenFlow switch according to NPL 2 has a secure channel for communication with an OpenFlow controller and operates according to a flow table suitably added or rewritten by the OpenFlow controller. In the flow table, a set of the following three is defined for each flow: match conditions (Match Fields) against which a packet header is matched; flow statistical information (Counters); and instructions that define at least one processing content (see section “5.2 Flow Table” in NPL 2).
For example, when an OpenFlow switch receives a packet, the OpenFlow switch searches the flow table for an entry having a match condition that matches header information of the received packet (see “5.3 “Matching” in NPL 2). If, as a result of the search, the OpenFlow switch finds an entry that matches the received packet, the OpenFlow switch updates the flow statistical information (Counters) and processes the received packet on the basis of a processing content(s) (packet transmission from a specified port, flooding, dropping, etc.) written in the Instructions field of the entry. If, as a result of the search, the OpenFlow switch does not find an entry that matches the received packet, the OpenFlow switch transmits an entry setting request to the OpenFlow controller via the secure channel. Namely, the OpenFlow switch requests the OpenFlow controller to transmit control information for processing the received packet (Packet-In message). The OpenFlow switch receives a flow entry that defines a processing content(s) and updates the flow table. In this way, by using an entry stored in the flow table as control information, the OpenFlow switch performs packet forwarding.
In a centralized-control-type network as typified by a network using the above OpenFlow, a control apparatus that controls switches in a centralized manner needs to recognize a topology of the links among the switches. For example, PTL 1 discloses an OpenFlow controller that includes a function of updating topology information by using information collected from an external apparatus and the OpenFlow switches by using the Link Layer Discovery Protocol (LLDP) standardized by IEEE802.1ab.